1. Field of the Invention
The present invention relates to twin screw extruders, and more particularly to an improved twin screw food extruder with bearing assemblies providing improved screw support to minimize the tendency of the extruder screws to come into wearing contact with each other and/or to the surrounding barrel walls during rotation. In addition, the bearing assemblies of the invention are preferably provided with means for the injection of an edible lubricant such as a liquid fat, so that the assemblies can be lubricated during use with liquids which enhance the final properties of the extrudate.
2. Description of the Prior Art
Extruders are industrial devices that are used to transport and process a wide variety of materials such as thermoplastic resins and plant-derived substances. Extruders generally include an elongated tubular barrel having a material inlet and a material outlet and one or more rotatable, flighted extrusion screws positioned within the barrel for transporting material therethrough.
One class of extruders includes the "twin screw" extruder, which has a pair of juxtaposed, elongated, flighted screws within a complemental barrel. An example of a twin screw extruder is described in U.S. Pat. No. 4,875,847, commonly owned with the present invention and hereby incorporated by reference.
One of the chief advantages of twin screw extruders, as compared with mono-screw extruders, is that twin screw extruders more efficiently transport material through the extruder barrel. This is because mono-screw extruders introduce fore and aft movement of the material as it progresses along the length of the barrel. Twin screw extruders avoid such fore and aft movement by the use of two intermeshed screws that operate in the manner of a positive displacement pump.
Despite the advantages inherent in the design of twin screw extruders, some prior art twin screw devices suffer from operational problems that significantly increase their cost and therefore limit their utility. Particularly, twin screw extruders can exhibit a marked tendency to prematurely wear out machine components. This is because the screws on prior twin screw extruders have a bearing support only at their rearmost or product inlet ends provided by their drive shafts, but are unsupported at their forward or material outlet ends.
This cantilever-type support is problematic because, when in use, twin screw extruders are subject to a build-up of pressures near the region where the screws are intermeshed. This pressure tends to separate and push the screws into wearing contact with the adjacent barrel walls, thus rapidly wearing both the screws and the barrel components.
In addition, conventional twin screw extruders with cantilever-type screw support can suffer wear problems because of inordinate screw-to-screw contact during extruder operation. This type of wear is especially acute during high rotational speed operation of the extruder screws.
The above-referenced '847 patent discloses a significantly improved extruder having conical nose screw sections adjacent the die ends of the screws which provide a bearing-type support during use of the extruder, thus reducing wear. The present invention provides a new improved twin screw extruder that even more effectively eliminates wear problems commonly encountered with conventional twin screw extruders.